It has been mentioned many times in this forum; the importance of Resistance in boss fights. I experienced it first hand in my impossible mage game in the fight against Baal. I had about 29k Leadership (including Flaming Eyes) and used an undead setup with Necromancers, Vampires, Ancient Vampires, Bone Dragons and Death Knights. I attacked with the vampires and the death knight. I had also put Stone Skin on the death knight. In one attack K'Tahu attacked my ancient vampire stack and destroyed it in one blow (Can bosses score a critical hit - this sure felt like one
). Later the same attack killed only 5 of my death knights (out of ca 200)...